Sec. 7. (a) Except as provided in subsection (b), a person who acted as an agent in the making or continuing of a contract with any person who has died, is not a competent witness, in any suit upon or involving the contract, as to matters occurring before the death of the decedent, on behalf of the principal to the contract, against the legal representatives or heirs of the decedent.
(b) If the person is called by the decedent's heirs or legal representatives, the person is a competent witness, as to matters about which the person is interrogated by the heirs or representatives.